The axe may be so firmly embedded in the wood that you raise the wood along with the axe. Don't worry, just raise them both and repeat the action. The force of the wood and axe hitting the chopping block should cause the wood to split.

It should also be noted** that during the activity of chopping wood small chips may fly off in unpredictable directions. It's also a good idea to move anything that may get damaged away from the area (including people!). My mate left his toolbox a little too near the chopping block, and paid the price when I accidentally put the axe through it. ;o)